Hi Greenboy,
Well.. You found the right place for all the info you need.
Welcome to the NZ Fishroom
First thing is that if yor setup is only three weeks old, then you should refrain from adding more fish at this point.
Take things VERY slowly, as the biggest mistake that newcomers to the hobby make is overstocking their tanks before they have had the time to become established... which could take several months.
The general rule is, "Big changes... big problems".. and overstocking your tanks in the early stages is heading for disaster, so just take your time and add a couple of fish every month or six weeks until your stock level approaches its peak.
?
This of course will depend on the size of the filter, the type, and the amount of work it has to do.
The filter needs to become the home for the good bacteria that are going to live there, so you have to treat it with great care.
Only "RINSE" the media in a container of old tank water, and never under the running tap, or using hot water, as this will immediately destroy the colony of bacteria it holds.
There's lots of topics and articles about this subject in the Beginners Section, or check my link below.
Great to have you with us, and hope you enjoy your visits.
Quite a few of the members are in the chat room on an evening around 9pm if you fancy a chat